Low-emissivity (low-E) coating systems are known in the art. For example, commonly owned U.S. Pat. No. 5,376,455 discloses: glass/Si3N4/NiCr/Ag/NiCr/Si3N4. Unfortunately, the low-E coating system of the '455 patent is not sufficiently color matchable after heat treatment (HT) with its non-heat treated counterpart.
The need for substantial matchability (before heat treatment vs. after heat treatment) is known. Glass substrates are often produced in large quantities and cut to size in order to fulfill the needs of a particular situation such as a new multi-window and door office building, vehicle window needs, etc. It is often desirable in such applications that some of the windows and/or doors be heat treated (i.e., tempered, heat strengthened or heat-bent) while others need not be. Office buildings often employ IG units and/or laminates for safety and/or thermal control. It is desirable that the units and/or laminates which are heat treated (HT) substantially match their non-heat treated counterparts (e.g., with regard to color, reflectance, and/or the like, at least on the side to be viewed from outside the building) for architectural and/or aesthetic purposes.
Commonly owned U.S. Pat. No. 5,688,585 discloses a solar control coated article including: glass/Si3N4/NiCr/Si3N4. One object of the '585 patent is to provide a sputter coated layer system that after heat treatment (HT) is matchable colorwise with its non-heat treated counterpart. While the coating systems of the '585 patent are excellent for their intended purposes, they suffer from certain disadvantages. In particular, they tend to have rather high emissivity and/or sheet resistance values (e.g., because no silver (Ag) layer is disclosed in the '585 patent).
It has in the prior art been possible to achieve matchability in systems other than those of the aforesaid '585 patent, between two different layer systems, one of which is heat treated and the other is not. The necessity of developing and using two different layer systems to achieve matchability creates additional manufacturing expense and inventory needs which are undesirable.
U.S. Pat. Nos. 6,014,872 and 5,800,933 (see Example B) disclose a heat treatable low-E layer system including: glass/TiO2/Si3N4/NiCr/Ag/NiCr/Si3N4. Unfortunately, when heat treated this low-E layer system is not approximately matchable colorwise with its non-heat treated counterpart (as viewed from the glass side). This is because this low-E layer system has a ΔE* (glass side) value greater than 4.1 (i.e., for Example B, Δa*G is 1.49, Δb*G is 3.81, and ΔL* (glass side) is not measured; using Equation (1) below then ΔE* on the glass side must necessarily be greater than 4.1 and is probably much higher than that).
U.S. Pat. No. 5,563,734 discloses a low-E coating system including: substrate/TiO2/NiCrNx/Ag/NiCrNx/Si3N4. Unfortunately, it has been found that when high Nitrogen (N) flow rates are used when forming the NiCrNx layers (see the high N flow rate of 143 sccm in Table 1 of the '734 patent; translating into about 22 sccm/kW), the resulting coated articles are not color stable with heat treatment (i.e., they tend to have high ΔE* (glass side) values greater than 6.0). In other words, if subjected to HT, the '734 patent low-E layer system would not be approximately matchable colorwise with its non-heat treated counterpart (as viewed from the glass side).
Moreover, it is sometimes desirable for a coated article to have high visible transmission characteristics and/or good durability (mechanical and/or chemical). Unfortunately, certain known steps that are taken to improve visible transmission characteristics and/or pre-HT durability tend to degrade post-HT durability and thermal stability. Thus, it is often difficult to obtain a combination of high visible transmission, thermal stability of color, and good durability.
In view of the above, it will be apparent to those skilled in the art that there exists a need for a low-E coating or layer system that after HT substantially matches in color and/or reflection (as viewed by a naked human eye) its non-heat treated counterpart. In other words, there exists a need in the art for a low-E matchable coating or layering system. There also exists a need in the art for a heat treatable system that can combine one or more of: (1) high visible transmission characteristics, (2) good durability before and/or after heat treatment, and/or (3) a low ΔE* value which is indicative of color stability upon heat treatment.
It is a purpose of this invention to fulfill one or more of the above-listed needs, and/or other needs which will become more apparent to the skilled artisan once given the following disclosure.